Implement construct_inputs for custom models

[thopkins32]

This enables their use in Ax generation strategies. The tutorial for how to do this is here: https://ax.dev/docs/tutorials/modular_botorch/#implementing-a-custom-model

[thopkins32]

See also: https://ax.dev/docs/generation_strategy/#input-constructors for a brief mention of how one would do this for MultiTaskGPs.